The size and morphology of primary grains are of particular importance for physic-chemical and mechanical properties of various iron-based materials such as austenitic grades stainless steels. A typical cast macro-structure of austenitic grade stainless steels consists of columnar zone formed by elongated dendrite crystals growing from externally cooled casting surfaces and an inner zone with equiaxed grains. The ratio of equiaxed to columnar structure may be, for example, on the order of 10:90 to 55:45, e.g., between 10 and 55 vol % equiaxed structure.
Grain refinement of cast structure in iron-based materials is an important tool for: (i) reducing compositional micro segregation within grains, (ii) decreasing the large scale of macro segregation of alloying elements within entire casting, and (iii) for control of structure and composition of the grain boundaries. In general, a fine equiaxed grain structure can lead to a more uniform response in heat treatment, reduced anisotropy and better properties compared to large columnar grains. Refining structure improves both alloy strength and ductility. In high alloyed steels, the homogeneity of a fine equiaxed grain structure is better than columnar zone with elongated dendrites. Such castings exhibit reduced clustering of undesirable features, such as micro-porosity and non-metallic inclusions. A small equiaxed grain structure is also preferred because it promotes resistance to hot tearing.
One approach to grain refinement in austenitic stainless steels and other alloys has heretofore been to introduce pre-existing particles into the melt. The goal has been to have solid particles dispersed throughout the liquid molten metal, so that when the metal solidifies, its solidification mechanism is biased toward forming grains initiated throughout the metal over forming grains initiated from the side walls of the mold. This method of grain refinement presents various challenges in that the pre-existing particles must be formed and incorporated into a so-called master alloy which is then incorporated into the overall melt. The master alloy alters the overall composition of the melt, so careful control is required to avoid pushing the melt composition out of its specified compositional range. Also, the master alloy requires additional energy to melt, and can therefore require raising the temperature of the overall melt.